Add support for Gradle Apt Plugin

I already have another package that uses annotation processing (immutables), and it doesn't play nicely with the documented Gradle solution for adding annotation processing.

immutables plugs in directly to the Gradle Apt Plugin (https://github.com/tbroyer/gradle-apt-plugin)

It would be nice if the CodeGen worked similarly for vertx.. I have it working with Warnings only via:

dependencies {
compileOnly "org.immutables:value:2.4.4:annotations"
apt group: 'org.immutables', name: 'value', version: '2.4.4'

compileOnly group: 'io.vertx', name: 'vertx-codegen', version: vertxVersion
apt group: 'io.vertx', name: 'vertx-codegen', version: vertxVersion
apt group: 'io.vertx', name: 'vertx-docgen', version: vertxVersion
apt group: 'io.vertx', name:'vertx-service-proxy', version:vertxVersion, classifier: 'processor'
}

This generates both sets of classes but my gradle output is littered with warnings:

warning: No SupportedSourceVersion annotation found on io.vertx.serviceproxy.ServiceProxyProcessor, returning RELEASE_6.
warning: Supported source version 'RELEASE_6' from annotation processor 'io.vertx.serviceproxy.ServiceProxyProcessor' less than -source '1.8'

I have java code and source compatibility set to 1.8 but can't figure out how to feed this in to avoid the warning. Hopefully this is just a documentation issue, would be helpful to add this as an option to the documentation for anyone else using the apt plugin.